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X-ray diffraction – the experiment

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1 X-ray diffraction – the experiment
Learning Outcomes By the end of this section you should: understand some of the factors influencing X-ray diffraction output be aware of some X-ray diffraction experiments and the information they provide know the difference between single crystal and powder methods

2 Methods and Instruments
All are based on: X-ray Source Sample Detector Sample can be: Single crystal Powder - (what is a powder?!)

3 X-rays - interactions First assumption: X-rays elastically scattered by electrons. Second assumption: Spherical, discrete atoms J. J. Thomson’s classical theory of X-ray scattering. X-ray output is defined through the scattering cross-section. where r0 is the classical electron radius. Very weak interaction. Thus need lots of electrons, and thus many atoms. J. J. Thomson, “Conduction of Electricity through Gases”

4 Scattering factor More electrons means more scattering ( Z)
Scattering per electron adds together, so helium scatters twice as strongly as H We define an atomic (X-ray) scattering factor, fj, which depends on: the number of electrons in the atom (Z) the angle of scattering

5 Function of deflection angle
f varies as a function of angle , usually quoted as a function of (sin )/ The more diffuse the electron cloud, the more rapid the reduction in the scattering function with scattering angle.

6 Deflection angle / atomic number
Different elements show the same trend: note the starting value (sin ) /

7 f  Z (ish) For  = 0, f is equal to the total number of electrons in the atom, so f=0 = Z Ca2+ and Cl- both have 18 electrons. So at =0 fCa = 18 = fCl But as  increases, Cl- has smaller f as it has a more diffuse electron cloud

8 What is important? Lots of scattering centres
Large enough crystals (lots of planes) Long range order (otherwise??) Glass crystallising with temperature Broad, featureless pattern. Some information can be retrieved (e.g. average atomic distances) but no structure.

9 Bragg (again!!) Look at Bragg set-up with different emphasis
hkl 1000’s of planes (1000Å = 1m) Scattering: angle and Z Thus the scattering from this plane will reflect which atoms are in the plane. Turn the crystal….

10 Bragg (again!!) Scattering: angle and Z hkl d expands
Changes d-spacing and atoms within the planes So we need to either (a) rotate the crystal or (b) have lots of crystals at different orientations simultaneously

11 Detector photographic film or area detector
Laue Method White X-ray source Collimator Fixed single crystal Detector photographic film or area detector Max Von Laue Nobel Prize 1914

12 Laue Method

13 Laue Method Each spot corresponds to a different crystal plane USES:
alignment of single crystal info on unit cell info on imperfections, defects in crystal Not so common these days…

14 4-circle Method Monochromatic X-rays Moving detector
Movingsingle crystal Crystal can be oriented so that intensities for any (hkl) value can be measured

15 Actual instrument

16 Now more common to use area detector which removes one circle.

17 Bruker SMART Area detector

18 Output List of hkl (each spot represents a plane) and intensity
1000’s of data points needed

19 Uses Unit cell determination
Crystal structure determination (primary method) We will come to the theory later on… We’ve also used ours to get information on vertebral disks!!

20 Powder Diffraction By “powder”, we mean polycrystalline, so equally we can use a piece of metal, bone, etc. We assume that the crystals are randomly oriented so that there are always some crystals oriented to satisfy the Bragg condition for any set of planes Monochr. X-rays Detector - Film Counter

21 Film - Debye Scherrer Camera
Camera radius = R

22 Debye-Scherrer Camera
Now obsolete! Peter Debye, Nobel Prize 1936

23 Counter - Diffractometer
Bruker D8 Advance detector X-ray tube sample

24 More detail

25 Not all are the same… X-ray tube Furnace Detector Sample Detector
Stoe Stadi/P

26 Output Plot of intensity of diffracted beam vs. scattering angle (2)

27 The Powder Pattern The whole pattern is a representation of the crystal structure Not like some other techniques like spectroscopy Next section we will examine the uses in more detail, then the details behind the pattern

28 Summary Diffraction experiments consist of a source, a sample and a detector Samples can be single crystal or “powder” (polycrystalline) Single crystal is a primary technique for structure determination Powder diffraction relies on a random orientation of (small) crystallites

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